linux-kernel - Re: [PATCH v4] kprobes: arm: enable OPTPROBES for ARM 32

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Message-ID: <53EEC59B.6060101@hitachi.com>
Date:	Sat, 16 Aug 2014 11:44:43 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>, ananth@...ibm.com,
	anil.s.keshavamurthy@...el.com, davem@...emloft.net,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	peifeiyue@...wei.com, lizefan@...wei.com
Subject: Re: [PATCH v4] kprobes: arm: enable OPTPROBES for ARM 32

(2014/08/16 10:38), Wang Nan wrote:
> On 2014/8/15 23:23, Masami Hiramatsu wrote:
>> (2014/08/12 13:56), Wang Nan wrote:
>>> +/* Caller must ensure addr & 3 == 0 */
>>> +static int can_optimize(unsigned long paddr)
>>> +{
>>> +	return 1;
>>> +}
>>
>> As we have talked on another thread, we'd better filter-out all stack-pushing
>> instructions here, since (as you said) that will corrupt pt_regs on the stack.
>>
>> Thank you,
>>
> 
> So we need to identify the replaced instruction. I think some improvement on
> arm instruction decoder is required, else we have to implement another (although simpler)
> decoder for memory accessing instructions.

Since arm32 already have instruction emulator, I guess it's not so hard, we can
start with using emulator code to find which one will change sp.

> In this situation we are talking about, we need the decoder identify the addressing
> information for str/stm instroction.

No, sp register must be always the top of stack, or the code is just broken (breaks
stack frame). So we need to identify the stm/str instructions which destination is
sp register.

> However, decoder can bring up more information such as
> instruction type, source/destnation registers, memory access pattern ...
> With such information, we can further optimize our trampoline code.
> For example: doesn't protect destnation registers, and for some (most of, I think) instruction,
> we can direct execute them like x86_64.

Yeah, direct execution may reduce the overhead much :). But anyway, since we need pt_regs,
we have to store all registers same as pt_regs.

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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